This invention relates to the use of 3M Cartesian polarizing beam splitter (PBS) films to make electronic projection systems that use color separation and recombination prisms (e.g. Philips Prisms) with very efficient, low f/# optical beams while preserving high contrast. More specifically, the present invention relates to an optical imaging system including a reflective imager and a Cartesian wide-angle polarizing beam splitter (“PBS”) having a fixed polarization axis and using the tilted reflective surfaces of a Philips prism.
Optical imaging systems may include a transmissive or a reflective imager or light valve. Traditional transmissive light valves allow certain portions of a light beam to pass through the light valve to form an image. By their very function, transmissive light valves are translucent; they allow light to pass through them only where required electrical conductors and circuits are not present. Reflective Liquid Crystal on Silicon (LCOS) imagers, in turn, reflect selected portions of the input beam to form an image. Reflective light valves provide important advantages, as controlling circuitry may be placed below the reflective surface, so that these circuits do not block portions of the light beam as in the transmissive case. In addition, more advanced integrated circuit technology becomes available when the substrate materials are not limited by their opaqueness. New potentially inexpensive and compact liquid color display (LCD) projector configurations may become possible by the use of reflective LC microdisplays. Reflective Liquid Crystal on Silicon (LCOS) imagers in the past have been incorporated into inefficient, bulky and expensive optical systems.
For projection systems based on reflective LCD imagers, a folded light path wherein the illuminating beam and projected image share the same physical space between a polarizing beam splitter (PBS) and the imager offers a desirably compact arrangement. A PBS is an optical component that splits incident light rays into a first polarization component and a second polarization component. Traditional PBS's selectively reflect or transmit light depending on whether the light is polarized parallel or perpendicular to the plane of incidence of the light: that is, a plane defined by the incident light ray and a normal to the polarizing surface. The plane of incidence also is referred to as the reflection plane, defined by the reflected light ray and a normal to the reflecting surface.
Based on the operation of traditional polarizers, light has been described as having two polarization components, a p-component or direction and a s-component or direction. The p-component corresponds to light polarized parallel to the plane of incidence. The s-component corresponds to light polarized perpendicular to the plane of incidence. A so-called MacNeille PBS will substantially reflect s polarized light incident on the PBS surface (placed along the diagonal plane connecting two opposing edges of a rectangular glass prism), and substantially transmit p polarized light incident upon this surface. Traditional MacNielle PBS technology, is known, and is described in, for example, U.S. Pat. No. 2,403,731; H. A. Macleod, Thin Film Optical Filters, 2nd Edition, McGraw-Hill Publishing Co., 1989; pp. 328–332.
To achieve the maximum possible efficiency in an optical imaging system, a low f/# system is desirable (see, F. E. Doany et al., Projection display throughput; Efficiency of optical transmission and light-source collection, IBM J. Res. Develop. V42, May/July 1998, pp. 387–398). The f/# measures the light gathering ability of an optical lens and is defined as:f/#=f (focal length)÷D (diameter or clear aperture of the lens)
The f/# (or F) measures the size of the cone of light that may be used to illuminate an optical element. The lower the f/#, the faster the lens and the larger the cone of light that may be used with that optical element. A larger cone of light generally translates to higher light throughput. Accordingly, a faster (lower f/#) illumination system requires a PBS able to accept light rays having a wider range of incident angles.
The maximum incident angle θmax (the outer rays of the cone of light) may be mathematically derived from the f/#, F:θmax=tan−1((2F)−1)
Traditional folded light path optical imaging systems have employed the previously described optical element know as a MacNeille PBS. MacNeille PBS's take advantage of the fact that an angle exists, called Brewster's angle, at which no p-polarized light is reflected from an interface between two media of differing index. Brewster's angle is given by:θB=tan−1(n1/n0),where n0 is the index of one medium, and n1 is the index of the other. When the angle of incidence of an incident light ray reaches the Brewster angle, the reflected beam portion is polarized in the plane perpendicular to the plane of incidence. The transmitted beam portion becomes preferentially (but not completely) polarized in the plane parallel to the plane of incidence. In order to achieve efficient reflection of s-polarized light, a MacNeille polarizer is constructed from multiple layers of thin films of materials meeting the Brewster angle condition for the desired angle. The film thicknesses are chosen such that the film layer pairs form a quarter wave stack.
There is an advantage to this construction in that the Brewster angle condition is not dependent on wavelength (except for dispersion in the materials). However, MacNeille PBS's have difficulty achieving wide-angle performance due to the fact that the Brewster angle condition for a pair of materials is strictly met at only one angle of incidence. As the angle of incidence deviates from this angle a spectrally non-uniform leak develops. This leak becomes especially severe as the angle of incidence on the film stack becomes more normal than the Brewster's angle. As will be explained below, there are also contrast disadvantages for a folded light path projector associated with the use of p and s-polarization, referenced to the plane of reflection for each ray.
Typically, MacNeille PBS's are contained in glass cubes, wherein a PBS thin-film stack is applied along a diagonal plane of the cube. By suitably selecting the index of the glass in the cube, the PBS may be constructed so that light incident normal to the face of the cube is incident at the Brewster angle of the PBS.
MacNeille-type PBSs reportedly have been developed capable of discrimination between s- and p-polarized light at f/#'s as low as f/2.5, while providing extinction levels in excess of 100:1 between incident beams of pure s or pure p polarization. Unfortunately, as explained below, when MacNeille-type PBSs are used in a folded light path with reflective imagers, the contrast is degraded due to depolarization of rays of light having a reflection plane rotated relative to the reflection plane of the central ray. As used below, the term “depolarization” is meant to describe the deviation of the polarization state of a light ray from that of the central light ray. As light in a projection system generally is projected as a cone, most of the rays of light are not perfectly parallel to the central light ray. The depolarization increases as the f/# decreases, and is magnified in subsequent reflections from color selective films. This “depolarization cascade” has been calculated by some optical imaging system designers to effectively limit the f/# of MacNeille PBS based projectors to about 3.3, thereby limiting the light throughput efficiency of these systems. See, A. E. Rosenbluth et al., Contrast properties of reflective liquid crystal light valves in projection displays, IBM J. Res. Develop. V42, May/July 1998, pp. 359–386, (hereinafter “Rosenbluth Contrast Properties”) relevant portions of which are hereby incorporated by reference.
Recently, Minnesota Mining and Manufacturing has developed a novel type of birefringent polymeric multi-layer polarizing film (“3M advanced polarizing film” or “APF”). Co-assigned and co-pending parent application U.S. patent application Ser. No. 09/312,917, describes the use of such a film as a polarizing beam splitter. European Patent Application EP 0 837 351 A2 attempts to utilize another 3M dual brightness enhancing film (DBEF), an early 3M multi-layer film material, in a projection display apparatus having a “wide angle” reflecting polarizer. Such reference refers to p and s differentiation and uses the 3M material as a common reflective polarizer. Moreover, while “wide-angle” performance is a widely recognized design goal, references to “wide-angle” are meaningless absent contrast limits and spectral leak reduction and teachings on how to achieve such a goal. The 3M product “DBEF” is a reflective polarizer with typical block direction leakages of 4 to 6 percent at normal incidence. At higher angles the leakage is somewhat reduced, but at 45 degrees the extinction is typically still a few percent. Contrast ratios when using DBEF typically will be limited to maximum values at or below 99:1 for white light. However, DBEF suffers from spectral leaks that reduce the contrast of certain color bands to as low as 25:1, depending on the nature of the illumination source and the exact DBEF sample. To obtain superior performance it is desirable that good screen uniformity and the absence of spectral leaks in the dark state accompany good average contrast in all relevant color bands.
There has been previous work with non-telecentric configuration, reported by Paul M. Alt in the Conference Record of the 1997 International Display Research Conference (p. M 19–28) and in the IBM Journal of Research and Development (Vol. 42, pp. 315–320, 1998). These systems, however, used conventional MacNeille PBS cubes rather than a Cartesian PBS, and achieved a contrast ratio of only 40:1 at f/5. The PBS and the color prism were used in an s-orientation.
The need remains for an optical imaging system that includes truly wide angle, fast optical components and that may allow viewing or display of high-contrast images. Furthermore, it is desirable to enable optical designs that minimize the size of individual components, such as the color separation prism.
A color separation prism receives the polarized beam of light and splits the beam, generally into three-color components. Color prisms and imagers naturally have an orientation, including a long axis and a short axis. Optical designers are presently constrained to one of two options. The first is to place the imager on the color prism such that the long axis of the imager is parallel to the long axis of the color prism exit aperture (to the imagers). This allows the use of the smallest possible color prism, but under this condition, if the tilt axes of the PBS and the color prism are kept parallel to each other, then the designer is constrained to build the projector in a tower configuration. Such a configuration places the longest dimension of the projector in a vertical orientation, which may be unsuitable for a variety of applications. The second option is to place the long axis of the imager along the short direction of the color prism exit aperture (to the imagers). This allows the use of more desirable low-profile projector configurations, wherein the longest dimension of the projector is horizontal. However, this requires that the color prism be made larger and therefore that the projection lens have a longer back focal length. Consequently, this configuration will require larger, heavier, and more expensive projection lens and color prism components.